Enormous quantities of organic isocyanates are produced and consumed annually. Isocyanates find use in the preparation of urethane foam, coatings, pesticides and the like. The majority of isocyanates sold in the United States and world-wide comprises toluene diisocyanate (TDI)--usually in 80:20 mixture of the 2,4- and 2,6-isomers--and methylenediphenyl diisocyanate (MDI, predominantly the 4,4'-isomer) in a mixture with its higher oligomers, polymethylenepolyphenylene isocyanate (PMPPI).
Commercially, the most widely used method for preparing aromatic isocyanates is the phosgenation route which comprises the reduction of a nitroaromatic compound to the aromatic amine followed by reaction with phosgene to yield the aromatic isocyanate. The phosgene reaction is usually conducted at elevated temperatures and pressures. The phosgenation route is long, energy intensive and must handle highly corrosive materials such as hydrogen chloride, chlorine, sulfuric acid and nitric acid, and toxic reagents and intermediates such as nitroaromatic compounds, aromatic amines, chlorine and phosgene. The materials used in the construction of the process equipment also tend to the exotic in order to avoid chloride corrosion.
A new route which offers an alternative to the chlorine and hydrogen chloride which are generated in the phosgenation process is the carbonylation of the corresponding nitroaromatic compound as taught, for example, in U.S. Pat. Nos. 3,461,149; 3,674,827 and 3,914,268.
U.S. Pat. Nos. 3,925,435; 3,931,106; 4,029,686; and 4,145,360 disclose the production of isocyanates by the thermal decomposition of furoxanes.
The rearrangement of nitrile oxides to isocyanates using sulfur dioxide as a catalyst is taught by G. Trickes and H. Meier, Angew. Chem., Int. Ed. Eng., 16, 555 (1977).
A. Rahman et al., Pakistan Journal of Scientific Research, Vol. 30, 91 (1978), disclose that the reaction of nitroethane with acetic anhydride/triethylamine in non-polar solvents like ether or benzene, yields dimethyl furoxane and that of phenyl nitromethane with acetic anhydride/triethylamine in ether or benzene yields diphenyl furoxane. The authors state that the formation of furoxane in these reactions is an evidence of the intermediacy of a nitrile oxide.
Nitrile oxides are prepared from primary nitroparaffins by a one-step reaction under mild conditions using phenylisocyanate as the dehydrating agent in the presence of catalytic amounts of triethylamine. See The Nitrile Oxides, Ch. Grundman and P. Grunanger, Springer-Verlag New York, 1971, p. 52.
The above references disadvantageously require at least a stoichiometric amount of dehydrating agent which is consumed in the process.
There is a need for an efficient non-chloride method for producing aromatic isocyanates from nitroalkanes since the carbonylation process is still long and energy intensive.
There is yet a need for a method for making aromatic isocyanates from nitro compounds in a basically one-step process.
Further, there is a need for the ability to generate aromatic isocyanates from nitromethyl aromatic compounds.
There is still a further need for a route that quickly and efficiently generates isocyanate precursors in advantageous oxidation states and utilizes inexpensive dehydration chemistry instead of multiple oxidation-reduction sequences to prepare isocyanates from nitroalkanes.